Tokkai Sho 63-280862 published by the Japanese Patent Office in 1988, for example, discloses control of a basic ignition timing of an engine which realizes minimum spark advance for best torque (MBT) in order to improve engine fuel-cost performance.
However, the MBT is not used in all operating regions of the engine, and in some of these regions, the basic ignition timing is retarded relative to the MBT. For example, in the low speed, high load region of the engine, the basic ignition timing is set to a value later than the MBT so as to prevent knocking. Also even when an idle switch is ON, the basic ignition timing is set later than the MBT in order to improve engine running stability under no load conditions.
When the basic ignition timing is set to a value later than the MBT, torque can be increased by advance correction of the ignition timing even when there is a drop in engine speed when the idle switch is ON. In other words, due to the advance correction of the ignition timing, the ignition timing approaches the MBT and torque increases. As a result of this torque increase, the engine rotation speed returns to a set idle rotation speed. The basic ignition timing is also set to a value later than the MBT so as to prevent increase in the discharge amount of hydrocarbons (HC).
Various torque-down controls are also practiced in order to stabilize idle running, to prevent speed change shocks, or to reduce the shock due to a sudden acceleration or transient surge backing immediately thereafter.
Describing speed change shocks, a step occurs in the torque transmitted to the drive wheels when there is a shift-up of a transmission. The magnitude of the speed change shock due to this step depends on driving conditions such as engine rotation speed or throttle valve opening at that time. The torque step is also larger the higher the speed or the greater the throttle opening.
To eliminate this torque step, the following control is for example performed.
When there is a shift-up such as during acceleration, a torque-down request signal is sent from an automatic transmission control unit to an engine control unit via a communications device. In response to the torque-down request signal, the engine control unit applies a retardation correction to the ignition timing according to a throttle valve opening and the engine speed, so that a torque-down due to this retardation correction cancels out a torque-up due to the shift-up.
However, the offset amount from the MBT of the basic ignition timing and the generated torque are not in a first order linear relation, and it is not easy to precisely correct the ignition timing so as to accurately satisfy any given torque-down request.
Moreover the torque-down amount when a given ignition timing correction is applied depends on the magnitude of the offset amount from the MBT of the basic ignition timing. For example, the ignition timing correction amount required to obtain the same torque-down is different when the basic ignition timing is equal to the MBT and when it is not.
For these reasons, it was not easy to apply an ignition timing correction amount to a torque-down amount.